Modern communication systems seek components having a high linearity in order to reduce intermodulation distortions. Intermodulation distortion can adversely affect the operation of multichannel systems utilizing closely spaced channel frequencies, f1, f2, f3, etc. When the current (I) voltage (V) characteristics of all the circuit components are perfectly linear, e.g., I=α1×V, where α1 is a constant coefficient, no frequency transformation and mixing occurs in the circuit. However, when some of the components have a nonlinear I-V relationship, intermodulation products appear in some or all of the signal channels. One of the most adverse types of intermodulation distortion are the third order distortions, which generate intermodulation products at frequencies corresponding to 2×f1±f2, 2×f2±f1, 2×f2±f3, etc. When the channel frequency separation is small, the third order intermodulation products generate signals within the channel frequencies, f1, f2, f3, etc., thereby creating intermodulation distortions.
The I-V nonlinearity responsible for the third order distortion comes from a term proportional to V3. In particular, when the actual I-V characteristic is approximated by the polynomial expression I=α1×V+α2×V2+α3×V3+ . . . , the α3 coefficient gives rise to the third order intermodulation distortions. Frequently, a level of third order intermodulation distortion can be characterized by calculating the third order intercept point (IP3). IP3 corresponds to a fictitious extrapolated input power level at which the power at a fundamental frequency equals the power at an intermodulation frequency (e.g., 2×f1±f2). For a single element, IP3 in dBm can be calculated by:IP3=10×log 10[4×α13/(3×α3)×103].
A more practical expression for the magnitude of IP3 comes from considering a nonlinear component connected into a circuit. FIG. 1 shows a simple illustrative circuit 2 comprising a nonlinear component 4 having an impedance Z connected in series between a signal source having the voltage VS and internal impedance Z0 according to the prior art. Typical examples of circuits including such a connection include microwave switches, attenuators, power limiters, etc., connected into a radio frequency (RF) or microwave transmission line. For the circuit 2, the IP3 in dBm can be calculated by the expression:
                              IP          3                =                  10          ×          log          ⁢                                          ⁢                      10            ⁡                          [                                                                                          Z                      0                                        ⁢                                          α                      1                      3                                                                            2                    ⁢                                          α                      3                                                                      ⁢                                  (                                      1                    +                                          2                      ⁢                                              Z                        0                                            ⁢                                              α                        1                                                                              )                                ×                                  10                  3                                            ]                                                          (        1        )            